Field of the Invention
The invention relates to the technology of an in-cell touch display, and more particularly to a method for eliminating/reducing image sticking of an in-cell touch display and a mobile device using the same.
Description of the Related Art
A conventional liquid crystal display is usually filled with liquid crystal molecules and composed of color or monochrome pixels disposed in front of a light source (e.g., a backlight source) or a light reflector. Each addressable pixel of the display comprises a liquid crystal unit disposed closest to two electrodes. An intensity of an electric field between the electrodes can be changed by configuring a voltage between the two electrodes. The intensity of the electric field makes the molecules in the liquid crystal unit present a specific direction (i.e., parallel to or perpendicular to the electric field, or at a certain angle therebetween) with respect to the intensity of the electric field. When the liquid crystal is combined with a polarizer in a proper direction, the liquid crystal unit actually acts as a light gate to allow a predetermined number of photons at positions corresponding to the pixels to be outputted to the display. Therefore, the display can generate various levels of gray scales (or various levels of red, green or blue in a color condition) by adjusting the voltage between the two electrodes.
If the voltage between the two electrodes is held constant within a prolonged time interval, the so-called “image sticking” phenomenon occurs. This condition tends to occur especially in the in-cell touch display, in which a common voltage electrode is cut. FIG. 1 shows a structure of a conventional in-cell touch display. Referring to FIG. 1, in order to possess the touch sensing and liquid crystal displaying effects in an in-cell touch display 100, a reference voltage electrode of the liquid crystal display, which is originally used, is cut into multiple touch sensing electrodes 101, and one frame period is divided into a display period and a touch sensing period. Each touch sensing electrode 101 is set to a display common voltage (typically a negative voltage) in the display period, and a display driving and touch integrated circuit 102 of the touch sensing electrode 101 transmits a touch signal to the corresponding touch sensing electrode 101 in the touch sensing period to detect whether the touch sensing electrode 101 is touched or not.
However, the touch sensing electrodes 101 are connected to different pins of the display driving and touch integrated circuit 102, and route lengths from the touch sensing electrodes 101 to the display driving and touch integrated circuit 102 are different. FIG. 2 is a schematic view showing how image sticking of the conventional in-cell touch display is caused. In order to describe the causes of image sticking of the in-cell touch display, only a first touch sensing electrode 201 and a second touch sensing electrode 202 are depicted in FIG. 2. The first touch sensing electrode 201 is electrically connected to the display driving and touch integrated circuit 102 through a first charge/discharge route R1, and the second touch sensing electrode 202 is electrically connected to the display driving and touch integrated circuit 102 through a second charge/discharge route R2.
Because the first charge/discharge route R1 and the second charge/discharge route R2 have different lengths, the resistance of the first charge/discharge route R1 is different from the resistance of the second charge/discharge route R2. An equivalent capacitance of the first touch sensing electrode 201 with respect to the ground may be regarded as the same as an equivalent capacitance of the second touch sensing electrode 202 with respect to the ground. Because the length of the first charge/discharge route R1 is longer than the length of the second charge/discharge route R2, the resistance of the first charge/discharge route R1 is greater than the resistance of the second charge/discharge route R2. Thus, the resistor-capacitor (RC) delay composed of the first charge/discharge route R1 and a first common electrode 201 is longer than the RC delay composed of the second charge/discharge route R2 and a second common electrode 202. So, even if P1 and P2 concurrently output a pulse signal SP1 and a pulse signal SP2, a signal SRC2 received by the second common electrode 202 is delayed by the time T2, and a signal SRC1 received by the first common electrode 201 is delayed by the time T1, T1−T2=ΔT. In the ΔT time, the voltage of the second common electrode 202 is lower than the voltage of the first common electrode 201.
Parasitic charges are accumulated between the neighboring first touch sensing electrode 201 and second touch sensing electrode 202 for a long time, so that the accumulated charges would cause the image sticking. FIG. 3 is a schematic view showing image sticking caused by the cutting of the reference voltage electrode in the conventional in-cell touch display. As shown in FIG. 3, a dashed-line portion 301 represents the grid-like image sticking caused by the above-mentioned reasons.